This invention relates generally to a process for fabricating masks or reticles having excellent surface cleanliness and integrity and more particularly to reticles or masks useful for printing images with extreme ultraviolet radiation.
Integrated circuits are presently manufactured using lithographic processes. By transmitting light through a patterned mask the light interacts selectively with an energy sensitive photoresist material deposited onto a substrate in such a way that a pattern or image is produced on the resist material. The resist material is developed and the pattern is transferred by etching onto the substrate by processes well known in the semiconductor art.
However, as integrated circuits have become smaller demands to achieve submicron resolution with sufficient line width control on a substrate have become increasingly important. Design rules of 0.5 .mu.m are being replaced by design rules that require feature sizes of 0.25 to 0.18 .mu.m and significant effort is presently being put into achieving 0.1 .mu.m resolution. As the feature size decreases, the wavelength of light required for submicron resolution decreases (for a design rule of 0.1 .mu.m light with a wavelength of about 13 to 15 .mu.m is preferred) with a corresponding increase in the photon energy. At these shorter wavelengths the light is strongly attenuated by a conventional transmission mask and thus projection methods using masks that reflect rather than transmit light become necessary.
Reflecting masks for extreme ultraviolet (EUV) or soft x-ray projection lithography, i.e., light whose wavelength is in the range 3.5-15 nm, are conventionally fabricated by depositing a reflective multilayer coating onto a substrate and patterning an absorber layer thereon. It should be noted that the term "reticle" is used to describe a mask having a pattern disposed thereon whose magnification is larger than that to be defined on the substrate. Typical geometry reductions range between four and ten times, the reticle features being greater than the actual printed image. Feature reduction allows complex circuitry to be imaged from the relatively large size reticles that are easier to make than masks having submicron features and makes mask defects easier to repair. Hereinafter the terms "mask" and "reticle" will be used interchangeably.
The multilayer coating used to reflect EUV can be composed of alternating layers of Mo and Si or Mo and Be deposited onto a polished silicon or quartz substrate. Various schemes for patterning the reflective multilayer coating have been reported, such as removal of selected areas of the reflective multilayer coating itself to form a surface having both absorbing and reflecting areas and depositing and patterning an overlaying absorber layer. In the latter case, rather than the conventional dry etching step, liftoff patterning can be used to limit damage to the underlying surface of the reflective multilayer coating. However, there are problems associated with this process such as generation of defects due to debris left on the reflective coating.
An alternative approach to fabricating masks has been disclosed in U.S. Pat. No. 5,304,437. Here a protective or barrier layer that can be silicon dioxide or an organic polymeric material such as a photoresist material can be deposited onto the multilayer coating to protect the surface of the reflective multilayer coating. A layer of material capable of absorbing the incident radiation (in the case of EUV, Au or Ge can be used) is deposited onto the protective layer. A top patterning layer of photoresist is then spun onto the absorber material. A circuit pattern can be produced on the surface of the resist using conventional steppers or e-beam writers. The circuit pattern is etched onto the absorber layer by conventional integrated circuit etching techniques finally, the top photoresist patterning layer is removed by an oxygen stripping etch and the underlying protective layer can typically be patterned by a more aggressive oxygen reactive ion etching (RIE) process.
It has been found in practice, however, that the absorber layer is prone to delaminate from the underlying protective layer, as shown in FIGS. 1a and 1b, particularly during the step of oxygen plasma stripping of the patterning photoresist layer. Furthermore, in the case of polymeric protective layers, outgassing of this underlying protective layer can take place which can cause undesirable bubbles to form in the absorber surface and can also lead to delamination of the absorber layer from the surface of the underlying protective layer. Moreover, the oxygen RIE process can damage the surface of the reflective multilayer coating.